The present invention relates to an image forming apparatus for forming an image by using an electrophotography technique, such as a printer, a facsimile, a copying machine or the like.
A related-art image forming apparatus is shown in FIG. 19. The apparatus comprises a photoreceptor (image supporting device) 1 having a photosensitive layer on the outer peripheral surface thereof, a charging device 2 for uniformly charging the outer peripheral surface of the photoreceptor 1, an exposing device 3 for selectively exposing the outer peripheral surface uniformly-charged by the charging device 2 to light to form an electrostatic latent image, a developing device 4 for applying toner serving as developer to the electrostatic latent image formed by the exposing device 3 to form a visible image (toner image), and an intermediate transfer belt 5 suspended among plural rollers 5a to 5d with tension. The toner image formed on the photoreceptor 1 is primarily transferred at a position T1 onto the intermediate transfer belt 5 and further secondarily transferred at a position T2 onto a recording medium. The related-art apparatus further comprises a fixing device 6 for heating and fixing the toner image concerned on the recording medium on which the toner image from the intermediate transfer belt 5 is transferred. These constituent elements are accommodated in a casing as shown in FIG. 19.
In the related-art image forming apparatus described above, all the rollers 5a to 5d among which the intermediate transfer belt 5 is stretched are disposed in the neighborhood of the center portion in the casing 7.
Therefore, the heat of the fixing device 6 is liable to bypass the intermediate transfer belt 5 and go around to the developing device 4 as indicated by an arrow A, so that the developing device 4 is easily heated.
Heating of the developing device 4 causes toner accommodated in the developing device 4 to be heated, and thus there is a problem that the transfer characteristic of the toner is deteriorated or the toner is liable to cause a blocking phenomenon in the developing device.
In the related-art image forming apparatus described above, the intermediate transfer belt 5 and the exposing device 3 are disposed to be far away from each other. Therefore, the heat of the fixing device 6 is liable to bypass the intermediate transfer belt 5 and go around to the developing device 4 as indicated by an arrow B, so that the developing device 4 is apt to be heated.
Heating of the developing device 4 causes toner accommodated in the developing device 4 to be heated, and thus there is a problem that the transfer characteristic of the toner is deteriorated or the toner is liable to cause blocking in the developing device.
In the related-art image forming apparatus described above, the fixing device 6 comprises an oil coating type fixing device. In FIG. 19, reference numeral 6a represents a fixing belt, and reference numeral 6b represents an oil coating roller for coating oil to the fixing belt 6a. 
In such a construction, oil is evaporated from both the surface of the fixing belt 6a and the surface of the oil coating roller 6b because the fixing device 6 is a heating type fixing device. The oil thus evaporated adheres to the surface of the intermediate transfer belt 5, resulting in occurrence of a problem that transfer failure is liable to occur or cleaning failure (filming) of the intermediate transfer belt 5 is liable to occur. The filming (the phenomenon that toner adheres or fixes in the form of a thin film) on the surface of the intermediate transfer belt 5 is further promoted by heating the toner in the developing device 4.
By the way, an optical sensor is used in such an image forming apparatus. For example, there is used an optical sensor for detecting the density of a toner image formed on the surface of the photoreceptor or the surface of the intermediate transferring member.
The optical sensor does not properly bring out its performance when the light emitting window or light receiving window thereof is soiled. When the image forming apparatus is operated, floating toner occurs inside the apparatus, and if the operating time is long, a large amount of floating toner would adhere to the light emitting window or light receiving window of the optical sensor, so that the performance of the optical sensor is remarkably deteriorated.
Accordingly, the light emitting window or light receiving window of the optical sensor is required to be cleaned before it is considerably soiled with toner.
However, it is not desirable to impose a cleaning work on a user. The imposition of the cleaning work on the user makes the user feel tangled, and it is not always that the cleaning is carried out by the user before the performance of the optical sensor is remarkably deteriorated.
Therefore, there has been proposed a cleaning device for automatically cleaning an optical sensor periodically (for example, at a rate of once per 30 to 40 times of the image forming operation) by moving a cleaner on the detection face of the optical sensor in synchronism with the operation of the image forming apparatus (disclosed in Japanese Patent Publication No. 5-82588B, for example).
However, in such a related-art apparatus, it is necessary to provide a complicated mechanism for moving such a cleaner in synchronism with the operation of the image forming apparatus.
Furthermore, since the optical sensor is automatically cleaned by the cleaner in synchronism with the operation of the image forming apparatus frequently (for example, at a rate of once per 30 to 40 times of the image forming operation), deterioration of the cleaner is promoted and finally no excellent cleaning effect can be achieved.
In a case where a charger (for example, a charger using corona discharging technique) generating ozone is used, and if ozone thus generated stagnates in a casing of the image forming apparatus, the ozone would adversely affect the surface of the image supporter and thus deteriorate an image to be formed.
Besides, when an image on the image supporter is developed by the developing device, toner sometimes scatters in the form of mist at the developing section (mainly at the downstream side in the rotational direction of the developing member which is the downstream side in the rotational direction of the image supporter), and the inside of the apparatus is polluted by the toner. In a case where the developing operation is carried out while switching the plural developing devices (developing colors) by intermittently rotating the rotator body. Therefore, toner is liable to scatter in the switching operation and this toner also pollutes the inside of the apparatus.
As a countermeasure to the ozone and the scattering toner as described above, it may be considered that a toner suction duct for mainly sucking scattering toner is formed at the downstream side of the developing portion with respect to the image supporter in connection with the rotational direction of the developing member, so that ozone as well as scattering toner is sucked by the toner suction duct.
However, it has been found that the following problem occur in such a construction. That is, when images having only a single color (for example, monochromatic images) are sequentially formed, the switching operation of the developing device is not carried out during the image forming operation, and a developing device for a specific color is kept to be in contact with or in close proximity to the image supporter, so that air flow directing from the upstream side to the downstream side in the rotational direction of the developing member is interrupted at the developing portion or extremely reduced.
Therefore, toner scattering to the downstream side in the rotational direction of the developing member is sucked by the toner suction duct, however, ozone stagnates at the upstream side in the rotational direction of the developing member, so that there occurs a problem that the stagnant ozone adversely affects the surface of the image supporter to deteriorate an image to be formed.